Active control of an acoustic cooling system

ABSTRACT

An acoustic cooling system comprising a first transducer ( 3 ) adapted for cooling by generating sound waves, a second transducer ( 4 ) adapted for cooling by generating sound waves, and a signal-processing unit ( 6 ) adapted to generate a cancellation signal for noise generated by the acoustic cooling system. The second transducer ( 4 ) is adapted to convert the cancellation signal into sound which at least partly cancels the noise.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a resonance cooling system, i.e. a cooling system that is brought to resonate, thereby causing a pulsating air stream that can be directed towards an object that is to be cooled.

BACKGROUND OF THE INVENTION

Today techniques exist where, for example, electrical components are cooled by means of an acoustic-resonance system. The most essential component of such a system is an acoustic transducer, e.g. a piezoelectric element, PVDF (polyvinylidine difluoride) material, a loudspeaker or any other electromagnetic or electrostatic transducer. When the transducer is connected to a resonator, such as an open resonant pipe or tube, or a Helmholtz resonator, a pulsating air stream is generated.

This air stream is used for cooling purposes, such as in electronic circuits and systems or in luminaries. The pulsating airflow is more effective in cooling than the laminar airflow obtained when employing more conventional cooling techniques.

The useful output of the acoustic-resonant coolers is the turbulences at the outlets of the resonators. These effect the actual cooling. Unavoidably, however, there will also be some remaining, more or less periodic air movements, which we perceive as sound, or more specifically as noise.

U.S. Pat. No. 6,625,285 discloses an acoustic cooling system with a noise reduction function. The cooling system has a temperature and pressure sensor that provides input values for driving an acoustic driver acting as a cooler. When the system is operated and cools a device, noise is produced which is detected by a microphone. The detected noise is then used for determining drive parameters for a loudspeaker that produces sound that cancels the noise produced by the system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvement of the above techniques and prior art.

A particular object is to provide an acoustic cooling system that efficiently may cool a warm object while the noise level of the system still is kept at a low level.

Hence an acoustic cooling system is provided, comprising a first transducer adapted for cooling by generating sound waves and a second transducer also adapted for cooling by generating sound waves. A signal-processing unit is adapted to generate a cancellation signal for noise generated by the acoustic cooling system, and the second transducer is adapted to convert the cancellation signal into sound which at least partly cancels the noise.

The inventive cooling system is advantageous in that noise is reduced without the need of incorporating any complex noise reducing means. Instead noise cancellation is performed by a component that also performs cooling. Preferably the sound waves for the respective transducer are generated in a fluid, and preferably the noise is completely cancelled out.

The acoustic cooling system may comprise a noise detector adapted to detect the noise generated by the acoustic cooling system, said noise detector providing a detected noise signal to the signal processing unit, which is advantageous in that adaptive noise cancelling may be achieved.

The signal-processing unit may be adapted to generate a predetermined cancellation signal, which is advantageous in that a more simple cooling system may be employed.

Each transducer may comprise a resonator, which is advantageous in that a desired resonance may be obtained for each pulsating airflow.

The second transducer may be excited by a signal which is derived by non-linear processing of a signal exciting the first transducer, which provides the possibility to handle nonlinearities of the transducers, i.e. it is possible to cancel noise having signal components at harmonics of the basic frequency of the drive values for the first transducer.

The driving power of the second transducer may substantially correspond to the driving power of the first transducer, which provides a simple and efficient control of the second transducer. It is understood that the driving power is the power that is used for generating the cooling exerted by the transducers.

The resonators may extend in essentially parallel directions, or the resonators may be arranged with a respective opening facing in essentially opposing directions, for providing versatile and flexible configurations of the cooling system.

According to another aspect of the invention, a method is provided of driving an acoustic cooling system, comprising the steps of: generating sound waves, by a first transducer adapted for cooling; generating sound waves, by a second transducer adapted for cooling; generating, by a signal processing unit, a cancellation signal for noise generated by the acoustic cooling system; and converting, by said second transducer, the cancellation signal into sound which at least partly cancels the noise.

The method may further comprise the step of detecting, by a noise detector, the noise generated by the acoustic cooling system.

The method may also comprise the step of providing a predetermined cancellation signal for the signal-processing unit.

The inventive method may further comprise the step of directing sound waves from each transducer by means of a respective resonator.

It should be noted that the inventive method may incorporate any of the features described above in association with the inventive cooling system, and has the same corresponding advantages. The sound generated by the transducers may of course have any suitable frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying schematic drawings, in which

FIG. 1 is a schematic drawing of a cooling system according to a first embodiment, and

FIG. 2 is a schematic drawing of a cooling system according to a second embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates an acoustic cooling system 1 with two transducers 3, 4. Each transducer 3, 4 generates sound waves for cooling and has a respective acoustic resonator 7, 8 with a respective opening 10, 11. In the embodiment of FIG. 1, the resonators 7, 8 are placed parallel to each other. A signal generator 13 sends a sinusoidal driving signal S1 to the first transducer 3, and the same signal S1 is sent to the second transducer via a non-linear circuit 14 and a signal processing unit 6.

The openings 10, 11 are typically arranged directed towards a warm object (not shown) that shall be cooled, and a temperature sensor (not shown) provides input values for the signal generator 13 for determination of the level of the driving signal S1, i.e. the level of cooling that shall be exerted by the system 1.

When the system 1 is operated, noise is usually generated and the noise is detected by a noise detector 5, or microphone. The detected noise is sent as a signal ε to the signal-processing unit 6.

The signal S1 sent from the signal generator 13 to the non-linear circuit 14 is transformed by the non-linear circuit 14 into a signal S′1 that contain harmonic frequency components of S1. S′1 may, for example, be a periodic pulse-shaped signal or “saw tooth” signal.

Of course, any suitable method for generating the signals S1 and S′1 may be employed, as long as sound derived from one of the signals may cancel out sound derived from the other signal.

For example, the two signals S1 and S′1 may be generated by retrieving values from a lookup table having values ranging from sin(0) to sin(2π). Each signal is associated with a respective pointer that repeatedly traverses the table, and since the pointers are divergent, phase shifted sinusoidal signals are generated. It is also possible to let one of the pointers traverse the table at, for example, twice the speed of the other pointer, which results in two harmonic signals.

In another version the two signals S1 and S′1 are generated by using a sinus/co sinus generator and by assigning a respective angular value to each signal S1, S′1. This renders it possible generate arbitrary phases for the signals by adapting only two input values. Of course, it is possible to use any other suitable trigonometric formula for generating phase shifted or harmonic signals S1, S′1.

The signal-processing unit 6 comprises an adaptive control element 9 for adaptively filtering S′1 by minimising the correlation between the noise signal ε and the signal S′1. The resulting signal S″1 is then used for driving the second transducer 4 which cancels the noise detected by the noise detector 5 as well as performs cooling.

In other words, the signal S′1 is filtered in an adaptive filter, the exact transfer function of which is determined by the microphone output signal ε. The criterion for adaptation is making the correlation between ε and S′1 as small as possible. This is a known procedure in the field of (either analog or digital) adaptive filtering. It can be performed in a variety of ways, which can be chosen from arbitrarily. By making ε as small as possible, the residue of the original acoustic excitation signal, which causes the unwanted noise, at the position of the microphone is minimized and unwanted noise is reduced considerably.

Due to the relatively large wavelength of the acoustic waves (typically for air 34 cm at 1 kHz), perfect noise cancellation at the exact position of the noise detector 5 will yield improved noise performance at greater distances from the resonator outlets 10, 11 as well.

Preferably the second transducer 4 “follow” the first transducer 3 in terms of cooling output. If, for any reason, the first transducer 3 starts to work harder, the second transducer 4 will automatically do the same. This means that if the cooling operation of the first transducer 3 is made dependent upon the actual temperature of the object to be cooled, e.g. via a temperature sensor, the second transducer 4 will automatically adapt its cooling operation accordingly.

In one version the noise detector 5 is omitted, in which case predetermined parameters for a noise-cancelling signal are stored in the signal-processing unit 6. The determination of these parameters is preferably performed during assembly of the cooling system 1, for example by using a temporary microphone at a specific distance from the system 1, and by determining the parameters so that noise at the microphone's position is cancelled out.

FIG. 2 illustrates a second embodiment where same components have the same reference numerals as in FIG. 1. Here the resonators 7, 8 are placed with their openings 10, 11 facing opposite directions, and the noise detector 5 is placed at the side of the resonators 7, 8.

In practise, the particular application of the cooling system 1, especially its physical configuration and the location of the object to be cooled, will determine which arrangement of the transducers/resonators is the most appropriate.

It should be noted that the principles of any suitable noise cancellation method may be employed without departing from the scope of invention.

Even if the system has been described as having two transducers, any suitable number of transducers may be employed, as long as at least one of them is configured to both cancel noise and provide cooling.

Preferably the two transducers 3, 4 generate sound waves in a fluid, such as air, but the sound waves may be generated in any suitable media. 

1. An acoustic cooling system, comprising: a first transducer adapted for cooling by generating sound waves, a second transducer for cooling by generating sound waves, and a signal processing unit for generating a cancellation signal for noise generated by the acoustic cooling system, said second transducer converting the cancellation signal into sound which at least partly cancels the noise.
 2. An acoustic cooling system according to claim 1, further comprising a noise detector configured to detect the noise generated by the acoustic cooling system, said noise detector providing a detected noise signal to the signal processing unit.
 3. An acoustic cooling system according to claim 1, wherein the signal processing unit is configured to generate a predetermined cancellation signal.
 4. An acoustic cooling system according to claim 1, wherein each transducer comprises a resonator.
 5. An acoustic cooling system according to claim 1, wherein the second transducer is excited by a signal derived by non-linear processing of a signal exciting the first transducer.
 6. An acoustic cooling system according to claim 1, wherein the driving power of the second transducer (4) substantially corresponds to the driving power of the first transducer (3).
 7. An acoustic cooling system according to claim 4, wherein the resonators extend in substantially parallel directions.
 8. An acoustic cooling system according to claim 4, wherein the resonators are arranged with a respective opening facing in generally opposing directions.
 9. A method of driving an acoustic cooling system, comprising the steps of: generating sound waves, by a first transducer adapted for cooling, generating sound waves, by a second transducer adapted for cooling, generating, by a signal processing unit, a cancellation signal for noise generated by the acoustic cooling system, and converting, by said second transducer, the cancellation signal into sound which at least partly cancels the noise.
 10. A method according to claim 9, further comprising the step of, by a noise detector, detecting the noise generated by the acoustic cooling system and providing a detected noise signal to the signal processing unit.
 11. A method according to claim 9, further comprising the step of providing a predetermined cancellation signal for the signal-processing unit.
 12. A method according to claim 9, further comprising the step of directing sound waves from each transducer by means of a respective resonator. 